Device for the cathodic protection of a metal wall against corrosion in a saline environment

ABSTRACT

A device for the cathodic protection of a metal wall against corrosion in a saline environment, includes an anode and means for connecting said anode to said wall. The anode has a higher electrochemical potential than the wall, wherein the anode is placed in a compartment delimited by a wall permeable to electrons and, optionally, to water. The device includes a porous outer layer made from a material selected from: polymeric materials, ceramic materials or hydrated inorganic materials and at least one porous layer having the ability to collect the cations emitted by the anode during the dissolution of same. The material forming the at least one layer is selected from osmotic membranes, active carbon, a cation exchange resin such as a zeolite, a cation-collecting polymer with nanofillers, cation-collecting mineral compounds such as phyllosilicates and inosilicates, cation-retaining nanofiltering semi-permeable organic microporous membranes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT/EP2013/051837 filed Jan. 31, 2013, which claims priority to French application 1250946 filed Feb. 1, 2012, both of which are hereby incorporated in their entireties.

TECHNICAL FIELD

The invention relates to the cathodic protection of installations submerged in a briny or saline environment. Protection of the metal hulls of boats or of metal parts against corrosion by seawater is often provided, conventionally, by a sacrificial anode, consisting of a metal that is more electronegative than that to be protected. When the boat and its sacrificial anode are immersed in seawater, polarization occurs, the hull becoming the cathode of an electrochemical cell and the sacrificial anode becoming the anode of this same cell. As a result, it is the metallic constituents of the sacrificial anode that are subjected to the effects of corrosion by the seawater and not those of the hull. These constituents M are released in the seawater in the form of cations according to the reaction M→M^(n+)+n e⁻.

The sacrificial anodes protecting the submerged steel parts of a boat or of any other seagoing vessel or fixed installation are generally of aluminum-zinc (2-6%)-indium (0.01-0.05%), or of aluminum-gallium (0.01%) or of zinc-Al alloy (0.1-0.5%). The drawback of this protection is that it releases the ions of the metals constituting the sacrificial anodes into the marine environment. Although this drawback is relatively minimal in the case of boats at sea, it has to be taken much more seriously when boats are berthed in a port, as the metals of the anode will accumulate in the water and in the sea bed of the berthing zones, where they will be absorbed by the creatures living there. The problem also arises for fixed installations such as oil rigs and off-shore wind turbines. It is therefore imperative to find effective and economical solutions for avoiding this release of harmful metals into the environment as far as possible, especially as developments in environmental protection legislation might make the use of solutions for controlling wastes from sacrificial anodes obligatory in certain circumstances.

An alternative solution to the use of a sacrificial anode as has just been described is to make this anode from a material that is not necessarily more electropositive than the material to be protected (which may be steel, cast iron, graphite, metal oxides, etc.), but apply an electric potential to it, constantly or cyclically, by means of a generator of direct or rectified current. This potential makes the anode more corrodible than the wall to be protected. This technique is onerous to apply, especially in zones of the installation with difficult access, but it is effective mainly for large installations. This technique is known as “impressed current cathodic protection” (abbreviated to ICCP).

SUMMARY

The aim of the invention is to propose a solution for avoiding release of the cations resulting from the dissolution of an anode of a device for cathodic protection into the environment.

For this purpose, the invention relates to a device for the cathodic protection of a metal wall against corrosion in a saline environment, comprising an anode and means for connecting said anode to said wall, said anode being at a higher electrochemical potential than said wall, characterized in that said anode is placed in a compartment delimited by a wall that is permeable to electrons and, optionally, to water, comprising:

-   -   a porous outer layer of a material selected from: polymer         materials, ceramic materials or hydrated inorganic materials;     -   and at least one porous layer able to capture the cations         emitted by the anode during dissolution thereof, the material         constituting said at least one layer being selected from osmotic         membranes, activated charcoal, a cation exchange resin such as a         zeolite, a cation capture polymer with nanofillers, cation         capture mineral compounds such as phyllosilicates and         inosilicates, semipermeable organic microporous nanofiltration         membranes of a type that retains cations.

Said wall may also comprise a membrane that traps negatively charged pollutants.

The anode may be a sacrificial anode whose electrochemical potential is naturally higher than that of the metal wall.

Otherwise, the device may comprise means by which an electrochemical potential higher than that of the metal wall can be applied to the anode.

The means for connecting the anode to the metal wall may be in contact with the wall outside the compartment and may pass through the porous wall of the compartment hermetically.

The means for connecting the anode to the metal wall may be in contact with the metal wall inside the compartment, and the porous wall of the compartment is connected hermetically to the metal wall.

The device may comprise means for keeping the anode at a distance from the metal wall.

It may comprise a plurality of layers able to capture the cations emitted by the anode during dissolution of the latter, each layer preferentially capturing cations different from those captured preferentially by the other layers.

As will have been understood, the invention consists of placing the anode in a compartment delimited by a series of porous membranes permeable at least to electrons, or even also to water, arranged in layers. The outer layer consists of a nonmetallic porous membrane, intended to reduce the hydraulic flow in the vicinity of the anode. The other porous layer or layers play(s) the role of cation barrier or trap, which prevents the cations resulting from dissolution of the anode to escape from the compartment into the environment.

If the membranes are all permeable to water, the space separating the anode from the wall of the compartment becomes filled with water naturally. If at least one of the membranes is not permeable to water but only to electrons, it is necessary, during installation of the device, to fill the compartment with water, preferably seawater to obtain good electrical conductivity, in order to bathe the anode in an electron-conducting medium and to endow the compartment with its operational form, with internal and external pressures that are balanced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the description given below, referring to the following appended figures:

FIG. 1, which shows schematically, in longitudinal section, a first example of carrying out the invention;

FIG. 2, which shows schematically, in longitudinal section, a second example of carrying out the invention.

DETAILED DESCRIPTION

FIG. 1 shows a metal wall 1 belonging to equipment installed in a marine environment 2, such as an oil rig or a wind turbine. However, this wall 1 could also be the hull of a boat, or of some other seagoing vessel. In a known manner, a sacrificial anode 3 is arranged in the vicinity of the wall 1, to which it is connected by electrical connectors 4, 5. In general, as in the prior art, the sacrificial anode 3 is constructed around a single steel connector, and in this case the connectors 4, 5 shown are in fact the two ends of this single connector. The material constituting the sacrificial anode 3 is conventional for this purpose (Al—In, Al—Ga or Zn alloy for example), and selection thereof is not a characteristic feature of the invention.

According to the invention, the sacrificial anode 3 is enclosed in a compartment 6 that is delimited by a set of membranes forming layers, and surrounds the anode 3 at a distance, for example of the order of 1 cm.

The outermost layer 7 is a porous layer permeable to electrons, and preferably also to water, intended to reduce the hydraulic flow between the external environment and the internal space 8 of the compartment. The material of which it is constituted is selected from polymer materials, ceramic materials or hydrated inorganic materials.

As examples of such materials, we may mention, nonexhaustively, thermoplastic polymers of the polyethylene or high-density polyethylene type, or porcelains of the industrial mullite or alumina type.

If this material is electrically insulating, it must be porous. In fact, polarization of the anode corresponds to establishment of a small electrochemical circuit, which can only function if electrons are circulating. The open porosity allows the electrons to pass into the liquid even if layer 7 is insulating.

This outermost layer 7, whose thickness is generally of the order of a millimeter, has the function of protecting the anode and the other membranes of compartment 6 against hydraulic abrasion. It must have suitable properties of wear resistance and impact strength, and resistance to deformation in the presence of a moving fluid.

The permeability of layer 7 is for example of the order of 10 ml/min per 1 cm² of anode surface.

The other layer or layers of the wall of compartment 6 (there are two of them, 9, 10, in the example shown) consist(s) of one or more materials serving as cation trap, which trap the cations emitted by the sacrificial anode 3 to prevent them entering the marine environment 2. Various types of materials may be suitable for this purpose: osmotic membranes, activated carbon in the form of powder or granules, a cation exchange resin such as a zeolite, a cation capture polymer with negative nanofillers attracting the cations, cation capture mineral compounds such as phyllosilicates and inosilicates. Such materials are included among those commonly used in water treatment and softening for cation capture or exchange. They may be supplemented with a membrane of activated alumina or of a functionally equivalent compound, which for its part traps the negatively charged pollutants, such as As and fluorides, which could reduce the efficacy of the membranes trapping the cations.

Semipermeable membranes employed in electrolytic processes of ion exchange may also be used.

Semipermeable organic microporous nanofiltration membranes of a type that retains cations may also be suitable.

The number of layers of cation capture materials is arbitrary, to be chosen by the user. These layers may advantageously be of multiple kinds, and each species of layer may, for example, preferentially absorb one or more of the chemical species that the sacrificial anode 3 is likely to release.

For example, we may envisage:

-   -   an outer layer 9 permeable to the chemical elements with radius         below 1.5 Å and impermeable to the chemical elements with radius         above 1.5 Å such as Ca, K, Mg, Na;     -   and an inner layer 10 permeable to the chemical elements with         radius below 1.1 Å such as O, Cl, N (which will therefore be         able to penetrate into the internal space 8 of compartment 6 or         leave it, which does not have drawbacks), and impermeable to the         chemical elements with radius above 1.1 Å such as Al, Zn, Ga,         In, thus the principal elements that the anode 3 may emit in the         form of cations, and whose release into the environment is         undesirable; the thickness of the layers, and notably of layer         10, may vary from about 1 mm to some cm, as a function of the         size of the anode 3 and therefore of the quantity of cations to         be trapped.

In the case when the whole of the wall delimiting compartment 6 is permeable to water, water penetrates into compartment 6 and a balance of pressures is attained between the interior and the exterior of compartment 6. Compartment 6 therefore assumes its nominal shape permanently and its wall is not subjected to crushing, which could lead to rupture thereof.

As has been mentioned, it is not obligatory for the whole of the wall defining compartment 6 to be permeable to water. It may only be permeable to electrons, but then prefilling of compartment 6 with water, preferably seawater, is necessary when installing the device according to the invention.

Owing to the invention, gradual dissolution of the sacrificial anode 3 takes place without pollution of the environment by the cations resulting from said dissolution, as they are captured by the layer or layers 9, 10. The latter must advantageously have a total capacity for absorption of the various cations and an absorption volume that are sufficient so that saturation does not occur before the end of the life of the sacrificial anode 3.

In the example shown in FIG. 1, the electrical conductors 4, 5 are in contact with the metal wall 1 in zones located outside of compartment 6. It is therefore necessary to ensure hermeticity of the wall of compartment 6 in the zones where the conductors 4, 5 pass through. However, as a variant, as shown in FIG. 2, the contacts between conductors 5, 6 and the metal wall 1 may be located within compartment 6. The wall of compartment 6 is then in hermetic contact with the metal wall 1 to be protected.

As shown in the figures, it is preferable that the sacrificial anode 3 is not in direct contact with the wall 1 to be protected. This avoids the creation of short-circuits between anode 3 and at least the zone of wall 1 that is opposite it. In this way, a larger part of the surface of wall 1 can be protected in the best conditions by one and the same anode 3. Means for keeping the anode 3 at a distance from wall 1 are therefore preferably provided (not shown in the figures). In practice, however, they can often consist of the connectors 4, 5, which are generally made of steel and have, owing to their material and their dimensions, sufficient rigidity to keep anode 3 at a distance from wall 1.

As a variant, the invention is also applicable to the case when the anode is not a sacrificial anode in the sense that it naturally has an electrochemical potential higher than that of the wall 1 to be protected, but is placed at this potential by a generator of direct or rectified current to which it is connected by conductors that pass through the wall of compartment 6 hermetically. 

1. A device for the cathodic protection of a metal wall against corrosion in a saline environment, comprising an anode and means for connection of said anode to said wall, said anode being at a higher electrochemical potential than said wall, wherein said anode is placed in a compartment delimited by a wall that is permeable to electrons and, optionally, to water, comprising: a porous outer layer of a material selected from: polymer materials, ceramic materials or hydrated inorganic materials; and at least one porous layer able to capture the cations emitted by the anode during dissolution of the latter, the material constituting said at least one layer being selected from osmotic membranes, activated charcoal, a cation exchange resin such as a zeolite, a cation capture polymer with nanofillers, cation capture mineral compounds such as phyllosilicates and inosilicates, semipermeable organic microporous nanofiltration membranes of a type that retains cations.
 2. The device as claimed in claim 1, wherein said wall also comprises a membrane that traps negatively charged pollutants.
 3. The device as claimed in claim 1, wherein the anode is a sacrificial anode whose electrochemical potential is naturally higher than that of the metal wall.
 4. The device as claimed in claim 1 wherein it comprises means for applying an electrochemical potential greater than that of the metal wall to the anode.
 5. The device as claimed in claim 1, wherein the means for connection of the anode to the metal wall are in contact with the wall outside the compartment and in that they pass through the porous wall of compartment hermetically.
 6. The device as claimed in claim 1, wherein the means for connection of the anode to the metal wall are in contact with the metal wall inside compartment, and in that the porous wall of compartment is connected hermetically to the metal wall.
 7. The device as claimed in claim 1, wherein it comprises means for keeping the anode at a distance from the metal wall.
 8. The device as claimed in claim 1, wherein it comprises a plurality of layers able to capture the cations emitted by the anode during dissolution of the latter, each layer preferentially capturing cations different from those captured preferentially by the other layers. 